Implantable heart monitoring device, system and method

ABSTRACT

In an implantable heart monitoring device and a monitoring method, an impedance is measured across at least part of an atrium, such that variation of the impedance is related to the volume change of the atrium. Values are stored at different occasions that indicate the rate of change of the measured impedance. The stored values are determined such that, when the device is used in a living being, the variation of the stored values will be related to the variation of the speed with which the atrium is filled with blood during the atrial diastole.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an implantable heart monitoring device,with which it is possible to monitor the heart condition. The inventionalso concerns a corresponding system and method.

2. Description of the Prior Art

Several different devices for monitoring the performance of a heart areknown. Often these devices are also able to deliver stimulation pulsesto the heart. The devices are often able to sense the electricalactivity in the heart. It is also known to determine an impedance valuemeasured between different electrodes positioned in or at the heart. Itis also known to sense other physiological parameters, such as pressure,oxygen level etc.

U.S. Pat. No. 5,154,171 describes the sensing of impedance values inorder to control the pacing rate.

U.S. Pat. No. 6,070,100 describes that electrodes may be positioned inor at both the left and the right atria as well as in or at the left andthe right ventricles. The document describes the possibility of sensingthe impedance between different electrodes. The sensed impedance valuesmay be used to improve the cardiac output.

United States Patent Application Publication No. 2001/0012953 describesbi-ventricular pacing. An impedance may be measured between electrodeson the right and the left sides of the heart. The variation of theimpedance with time is detected. The detected impedance variation may beused in order to synchronise the contraction of the ventricles.

United States Patent Application No. 2001/0021864 describes differentmanners of using the proximal and distal electrodes of different leadsin order to inject a current and to measure an impedance. The measuredimpedance value may be used in order to maximise the cardiac flow.

U.S. Pat. No. 6,314,323 describes a heart stimulator in which thecardiac output is determined by measuring the systolic pressure.

It is also known to communicate with an implanted heartmonitoring/stimulating device in a wireless manner, i.e. with the use ofso-called telemetry. This can be done by inductive communication or viaradio waves. With the help of telemetry it is thus for example possibleto obtain information about the status of an implanted device. It isalso known to input new information into the device with the help ofsuch telemetry.

For different severe cardiac conditions it may be important to monitorthe status of the heart, for instance in order to follow the progress ofa heart disease, for example in order to be able to carry out a suitabletreatment of the patient. One such heart condition is congestive heartfailure (CHF). This is a condition in which the heart's function as apump to deliver oxygen rich blood to the body is inadequate to meet thebody's needs.

Another concept used in this field is pre-load. Pre-load can be definedas the initial stretching of the cardiac myocytes prior to contraction.The concept of pre-load can be applied to either the ventricles oratria. Regardless of the chamber, the pre-load is related to the chambervolume just prior to contraction.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an implantable heartmonitoring device with which it is possible to monitor the status of theheart of a patient who suffers from a heart deficiency, such as CHF. Afurther object is to provide such a device with which it is possible todetect changes in the heart condition at an early stage. Another objectis to provide such a device, which with quite simple means makes itpossible to monitor the status of the heart condition of a patient whosuffers from such heart deficiencies.

The above objects are achieved in accordance with the present inventionby an implantable heart monitoring device and system, and a method,wherein an impedance is measured in vivo across a portion of the heartthat includes at least a part of an atrium of the heart, and such thatthe variation of the impedance is related to volume change of theatrium. Further in accordance with the device, system and method, acontrol circuit is configured to operate in time cycles that correspondto heart cycles of the heart and to, at a first occasion (A) monitor howthe measured impedance varies during at least a portion of a time cycle,and (B) determine a value that indicates the rate of change of themeasured impedance during at least a part of this portion of the timecycle, and to store that value in a memory. The control circuit isfurther configured to, at a number of further occasions, (C) repeat (A)and (B) so that, at each of the further occasions, a new determinedvalue, that indicates the rate of change of the measured impedanceduring the part of the time cycle, is stored in the memory. The valuesstored in the memory thus represent a record of the variation of thespeed with which the atrium is filled with blood during atrial diastole.

When the heart condition becomes worse, the heart will not be able topump away the returning blood to a sufficient degree, which among otherthings may result in pulmonary congestion. This will most likely have asa consequence that the (atrial) pre-load as well as the end (atrial)diastolic volume increase. Because of the higher pre-load and the largerend atrial diastolic volume, the atrial walls will be likely to be undermore tension and also dilated to a certain extent. Furthermore, moreblood will hereby remain in the atrium after the atrial systole (i.e.the atrium will have a higher residual volume). These conditions willhave as a consequence that, during the atrial diastole, the atrium willbe filled more slowly with new blood.

The rate of change of the measured impedance, in particular when theimpedance decreases, is an indication of how fast the atrium in questionis being filled with blood during the atrial diastole. With the presentinvention it is thus possible to monitor the status of the heart bymonitoring the mentioned impedance. A worsening of the heart conditionis likely to be first noticeable in the atrium. Consequently, with thepresent invention such a worsening of the heart condition can bedetected at an early stage. Moreover, since impedance measurement caneasily be provided by an implantable heart monitoring device (forexample an implantable cardiac rhythm management device), the inventioncan be implemented in a quite simple manner.

Preferably, the control circuit is configured to measure the impedancewith suitable electrode surfaces such that, when the device is used in aliving being, it is the impedance over the left atrium that will bemonitored. When the left atrium is monitored, a particularly relevantindication of the progress of the heart condition is obtained.

It should also be noted that preferably, at each occasion, it is therate of change during the same part of the respective time cycle that isdetermined.

According to an embodiment of the device according to the invention, thecontrol circuit is configured such that said rate of change of themeasured impedance is the rate of change when the impedance decreases.Since the impedance across the atrium decreases when the atrium is beingfiled with blood, this decreasing impedance is of course a particularlyrelevant indication of the filling phase of the atrium.

The stored value may be the absolute of the rate of change (i.e. whenthe impedance decreases, and the rate of change therefore is negative,the stored value may be the rate of change multiplied by −1).

The stored value may be for example the maximum absolute value of thederivative of the impedance as a function of time when the impedancedecreases, i.e. the maximum of

${\frac{\mathbb{d}Z}{\mathbb{d}t}}.$Another alternative is for example to form the value as:

${\frac{Z_{\max} - Z_{\min}}{t}},$where Z_(max) is the maximum value of the impedance, preferably themaximum following immediately after a detected QRS, Z_(min) is thesubsequent minimum value of the impedance and t is the time betweenZ_(max) and Z_(min). It is of course not necessary to consider theabsolute value of the first derivative. Instead, the minimum of thederivative of the impedance (when it decreases) can be the value that isstored.

Preferably, the control circuit includes cardiac sensing and/or pacingcircuits for enabling sensing of cardiac events of one or more atria orventricles of the heart and/or for pacing one or more atria orventricles. Such circuits are advantageous, since the heart can therebybe sensed and/or paced.

According to a further embodiment of the device, the control circuit isconfigured to be able to detect a QRS in a signal sensed from the heartand to determine whether the impedance signal decreases within a shorttime interval after the detected QRS, such that a major dip in thesensed impedance takes place after the detected QRS, such that themeasured impedance is likely to actually reflect the inverse of theamount of blood in an atrium.

For example, the onset of the QRS may be detected (the QRS is theventricular depolarisation seen in the sensed signal). If the impedancedecreases substantially directly after the (onset of the) QRS, this isan indication of the fact that it is actually an atrium that ismonitored. If, on the other hand, the impedance increases substantiallydirectly after the (onset of the) QRS, this is an indication of the factthat it is more likely that it is a ventricle that is monitored. If thelatter is the case, the control circuit can be configured not to use themeasured impedance, since it does not reflect an atrium, which it isintended to monitor.

It should be noted that the detection of the occurrence of the QRS inrelation to the measured impedance is just one advantageous manner ofdetermining that the measured impedance actually reflects when theatrium is being filled with blood. Also other alternatives are possible.The idea is that by determining in which part of the heart cycle theimpedance is measured, and by observing that the measured impedanceactually decreases as expected during the relevant part of the heartcycle, it can be determined that it is very likely that it is an atriumthat is being monitored. This can however be done in other manners thanby detecting the QRS. Instead, or additionally, other indications of thedifferent parts of the heart cycle can be used. It is for examplepossible to monitor the pressure in a heart chamber or the movement of aheart wall in order to determine in which part of the heart cycle theimpedance is being measured. It is also possible to detect the operationof the ventricles in different manners in order to determine the part ofthe heart cycle. For example, it is possible to monitor the impedanceacross at least one ventricle in order to detect when the ventricularsystole is taking place. If another measured impedance (the impedancethat is supposed to relate to the atrium in question) actually decreasesas expected during the ventricular systole, this is an indication of thefact that it is actually the atrium that is being monitored with thelatter impedance measurement.

According to a further embodiment, the control circuit is configured toselect said occasions such that the living being is likely to be in asimilar physical and/or psychological state at the different occasions.An accurate indication of the progress of the heart condition can beobtained if it is ensured that the patient is in a similar state at thedifferent occasions.

According to a further embodiment, the control circuit is configured tobe able to determine the physical and/or psychological state by one ormore of the following:

i) a physical activity sensor in the implantable heart monitoringdevice,

ii) the heart rate sensed or paced with the use of the implantable heartmonitoring device,

iii) a sensed breathing of the living being,

iv) a sensed posture of the living being, for example whether the livingbeing is standing up or laying down,

v) the time of day.

With such measures, the state of the patient can be determinedautomatically.

According to a further embodiment, the control circuit is configuredsuch that at each of said occasions, steps A and B are carried out anumber of times and such that from these number of times arepresentative value that indicates said rate of change at the occasionin question is formed, and wherein it is this representative value thatis stored in the memory at the occasion in question. By determining therate of change of the measured impedance a plurality of times, a moreaccurate representative value can be obtained.

According to an embodiment, said plurality of times takes place within 1hour, preferably within 5 minutes, more preferred within 1 minute.Thereby an accurate representative value can be formed that representsthe rate of change of the measured impedance at the occasion inquestion.

The representative value that is stored may for example be the meanvalue of 30 measurements, performed within two minutes. According to oneoption, values that deviate too much from predefined normal values maybe ignored.

According to another embodiment, the control circuit is configured tocommunicate with three or more electrode surfaces and also configured tobe able to carry out a procedure that involves testing differentcombinations of electrode surfaces used for injecting a current into theliving being in question and/or testing different combinations ofelectrode surfaces used for sensing the voltage there between, in orderto determine an optimal combination of electrode surfaces with which anoptimal impedance measurement for indicating the volume change of theatrium is obtained. By this measure, the impedance measurement may beimproved, since an optimal combination of electrode surfaces is used.

According to a further embodiment, the control circuit is configured todetermine said optimal combination of electrode surfaces based on one ormore of the following criteria:

the strength of the detected signal,

the ratio signal/noise of the detected signal,

the fact that the impedance signal decreases in such a part of the timecycle that the decreasing signal is likely to represent the filling ofthe atrium with blood.

In this way, the optimal combination of electrode surfaces can bedetermined in a quite straight forward manner. In order to determinewhether the last mentioned criteria is fulfilled, the control circuitcan be arranged at explained above, i.e. for example in order todetermine whether the decreasing impedance follows directly after adetected QRS.

According to a further embodiment, the control circuit is configuredsuch that said different occasions are distributed over at least severaldays, preferably over several weeks, or months, such that the variationof the stored values represents the variation in the detected rate ofchange of the measured impedance during a longer period. The long termchange of the heart condition thus can be monitored.

According to a further embodiment, the control circuit is configured tocreate a warning message if the stored values change more than apredetermined amount over a predefined time period.

The warning message may for example be created if the stored (absolute)value that represents the rate of change of the impedance has decreasedmore that 20% since the first (absolute) value was stored, i.e. comparedto the (absolute) value stored at the first occasion. According toanother example, the warning message may be created if the stored(absolute) value that represents the rate of change of the impedance hasdecreased more that 10% within two days. The warning message may forexample be that a warning is stored in the memory in order to alert aphysician at the next check-up. Another possibility is that the warningmessage directly alerts the patient carrying the device.

According to another embodiment, the control circuit is configured suchthat a plurality of said occasions, preferably at least 10 occasions,takes place during 24 hours such that the variation of the stored valuesrepresents the variation in the detected rate of change of the measuredimpedance during a 24 hour period.

Moreover, the control circuit may be configured to carry out thisprocedure at a plurality of different days and to determine, for eachday, the variation in the detected rate of change of the measuredimpedance during each 24 hour period, and to also determine how thedetermined variation changes from day to day.

According to an embodiment, the control circuit is configured to createa warning message if the change, from day to day, of the determinedvariation, in the detected rate of change of the measured impedanceduring a 24 hour period, fulfils a predetermined criteria.

The criteria may for example be that the determined variation, in thedetected rate of change of the measured impedance during a 24 hourperiod, has decreased more than 20% since the previous determinedvariation in the detected rate of change of the measured impedanceduring a 24 hour period. The rate of change is likely to vary to acertain degree during a day (during a 24 hour period). If this is notthe case, this may be an indication of the fact that the heart conditionhas become worse.

According to a further aspect, the invention provides an implantableheart monitoring system, comprising an implantable heart monitoringdevice according to any one of the preceding embodiments and at leasttwo of the electrode surfaces adapted to be positioned in a heart or inrelation to a heart of a living being, wherein the control circuit isset up to communicate with said electrode surfaces.

Although the control circuit could communicate with the electrodesurfaces in a wireless manner, the implantable heart monitoring systempreferably also comprises leads, which carry the electrode surfaces andwhich are adapted to be physically connected to the implantable heartmonitoring device.

With such an implantable heart monitoring system, advantagescorresponding to those described in connection with the implantableheart monitoring device are achieved.

According to a further aspect, the invention provides a method ofmonitoring a heart that proceeds as described in connection with theoperation of the implantable heart monitoring device. Such a methodbrings about advantages corresponding to those described in connectionwith the implantable heart monitoring device.

In analogy with the operation of the described device, also in themethod, in order to monitor when the atrium is being filled with blood,it is the rate of change of the measured impedance when the impedancedecreases that is considered. Also in the method it is preferably theimpedance over the left atrium that is measured, such that it is thefilling of the left atrium with blood that is being monitored.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an implantable heart monitoring device withleads and electrodes positioned in or in relation to a heart.

FIG. 2 shows schematically a control circuit and a memory which arecomprised in the heart monitoring device.

FIG. 3-5 shows examples of measured impedance as a function of time.

FIG. 6 shows schematically an example of how the rate of change of themeasured impedance may vary over time.

FIG. 7 is a schematic flow chart illustrating a method according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically an embodiment of an implantable heartmonitoring device 10 according to the invention. The device 10 has acasing 12, which may also function as an electrode surface in connectionwith for example impedance measurements. The device 10 has a controlunit 14, which controls the operation of the device 10. The device 10also comprises a memory 15 connected to the control circuit 14.Furthermore, the device 10 has an activity sensor 16 for sensing howphysically active the living being that carries the device 10 is. Thesensor 16 is connected to the control circuit 14.

The device 10 has a connector portion 13, via which the device 10 can beconnected to different leads 20, 30, 40, 50. The leads 20, 30, 40, 50are provided with electrode surfaces 21, 22, 31, 32, 41, 42, 51, 52, 53,54. The electrode surfaces 21, 31, 41, 51 are so-called tip electrodes,while the other electrode surfaces 22, 32, 42, 52, 53, 54 are so-calledring electrodes.

The device 10 together with the leads 20, 30, 40, 50 and the mentionedelectrode surfaces together constitute an embodiment of a systemaccording to the invention.

It should be noted that the device (and the system) may have many morecomponents and functions which are normal for a heart monitoring andpacing device.

The implantable heart monitoring device 10 is preferably also set up tobe able to sense the electrical activity of the heart and to pacedifferent heart chambers. In the shown embodiment, the lead 20 has beenintroduced into the right atrium RA such that the electrode surfaces 21,22 are positioned in this atrium. The lead 30 has been introduced intothe heart such that the electrode surfaces 31, 32 are positioned in theright ventricle RV. This lead 30 also has a pressure sensor 33 forsensing the pressure in the right ventricle RV. The electrode surfaces21, 22 can thus be used to sense and pace the right atrium RA and theelectrode surfaces 31, 32 can be used to sense and pace the rightventricle RV.

The lead 40 has been introduced via the right atrium RA and the coronarysinus such that the electrode surfaces 41, 42 are positioned in a veinnext to the left atrium LA. Similarly, the lead 50 has been introducedvia the right atrium RA and the coronary sinus such that the electrodesurfaces 51-54 are positioned in a vein next to the left ventricle LV.The different electrode surfaces 41, 42, 51-54 can thus be used to paceand sense the left atrium LA and the left ventricle LV in a manner knownto a person skilled in the art. In this example, the lead 50 has fourdifferent electrode surfaces 51-54 which make it possible to choosewhich electrode surfaces are to be used for sensing and pacing.

It is also well-known to a person skilled in the art that differentelectrode surfaces can be used for injecting a current and for sensing avoltage in order to measure an impedance across at least a portion ofthe heart. Also the casing 12 can be used for this purpose.

FIG. 2 shows schematically in particular the control circuit 14 in somemore detail. The control unit 14 has a control circuit 18 that controlsthe overall operation of the control unit 14. The control circuit 18 isconnected to be above mentioned memory 15. Furthermore, as is known tothose skilled in the art, the control unit 14 may comprise a sensingcircuit 25 and a pacing circuit 27, which circuits are adapted to beconnected to the lead 20 in order to pace and sense the right atrium RA.Moreover, a sensing circuit 35 and a pacing circuit 37 are adapted to beconnected to the lead 30 in order to sense and pace the right ventricleRV. Furthermore, a sensing circuit 45 and a pacing circuit 47 areadapted to be connected to the lead 40 in order to sense and pace theleft atrium LA. A sensing circuit 55 and a pacing circuit 57 are adaptedto be connected to the lead 50 in order to sense and pace the leftventricle LV. The different sensing and pacing circuits are of coursealso connected to the control portion 18. Although not shown, thecontrol unit 14 may be adapted to be connected to different ones of theelectrode surfaces 51-54 arranged on the lead 50 in order to make itpossible to select which of the electrode surfaces 51-54 that are to beused. Of course, also the other leads 20, 30 and 40 may be provided withmore or less than two electrode surfaces.

The control unit 14 is also configured to communicate with a number ofelectrode surfaces 12, 21, 22, 31, 32, 41, 42, 51, 52, 53, 54 and tomeasure an impedance with the help of at least two such electrodesurfaces 12, 21, 22, 31, 32, 41, 42, 51, 52, 53, 54. The impedanceindicates preferably the impedance across a portion of the heart thatincludes at least a part of the left atrium LA. How to measure such animpedance is known to a person skilled in the art, for example from someof the above-mentioned documents. For example, the control unit 14 canbe configured to inject a current between the electrode surfaces 22 and52 and to measure a voltage between the same electrode surfaces 22, 52.However, many other combinations of electrode surfaces can be used forthe impedance measurement, and different electrode surfaces may be usedfor injecting a current and for measuring a voltage. However, thecontrol circuit 14 is preferably set up such that the variation of themeasured impedance is related to the volume change of an atrium,preferably the left atrium LA.

The control unit 14 is configured to operate in time cyclescorresponding to heart cycles. This is normal for an implantable heartmonitoring or pacing device. The control unit 14 is configured to carryout the following steps: at a first occasion,

A) monitor how the measured impedance varies during at least a portionof a time cycle, and

B) determine a value that indicates the rate of change of the measuredimpedance during at least a part of said portion of the time cycle, andstore the value in the memory 15, and

at a number of further occasions,

C) repeat steps A and B, such that at the different occasions a newdetermined value, that indicates the rate of change of the measuredimpedance during said part of the time cycle, is stored in the memory15.

The control unit 14 is configured such that the values that are storedin the memory 15 have been determined such that, when the device 10 isactually used in a living being, the variation of the stored values willbe related to the variation of the speed with which the atrium is filledwith blood during the atrial diastole.

The control unit 14 is configured such that the rate of change of themeasured impedance is the rate of change when the impedance decreases.This means that the impedance is measured when the atrium LA is beingfilled with blood.

In order to make sure that the measured impedance actually reflects theamount of blood in the atrium, the control unit 14 is configured tocarry out the impedance measurement during a certain portion of theheart cycle (i.e. a portion of the mentioned time cycle with which thedevice operates). This can for example be done by detecting a QRS in thesignal sensed from the heart. The control unit 14 is configured todetermine whether the measured impedance decreases within a short timeinterval after the detected QRS, such that a major dip in the sensedimpedance takes place after the detected QRS. This means that themeasured impedance is likely to actually reflect the inverse of theamount of blood in an atrium.

FIG. 3-5 illustrate examples of how the measured impedance Z (in fact anormalised value that indicates how the impedance varies) as a functionof time t in seconds. The time 0 in the figures is the time when the QRSis sensed. The QRS is symbolised by the line 61. As can be seen in FIG.3 and FIG. 4, the measured impedance signal decreases rapidly directly,or shortly, after the QRS 61. This is an indication of the fact that themeasured impedance actually reflects the amount of blood in the atrium.

In FIG. 5, on the other hand, the impedance increases rapidly directlyafter the sensed QRS 61. This is an indication of the fact that in FIG.5, the measured impedance is more likely to reflect a ventricle than anatrium.

This may be caused by the fact that the electrode surface (for example52), which is one of the electrode surfaces used for the impedancemeasurement, is positioned so far down along the left ventricle LV thatthe measured impedance mainly reflects the left ventricle LV.

The control unit 14 is preferably configured to test differentcombinations of electrode surfaces 12, 21, 22, 31, 32, 41, 42, 51, 52,53, 54 used for injecting a current and for measuring a voltage in orderto determine an optimal combination of electrode surfaces 12, 21, 22,31, 32, 41, 42, 51, 52, 53, 54 with which an optimal impedancemeasurement for indicating the volume change of the atrium LA isobtained. The control circuit 14 may hereby be configured to determinethe optimal combination of electrode surfaces 12, 21, 22, 31, 32, 41,42, 51, 52, 53, 54 based on one or more of the following criteria:

the strength of the detected signal,

the ratio signal/noise of the detected signal,

the fact that the impedance signal decreases in such a part of the timecycle that the decreasing signal is likely to represent the filling ofthe atrium LA with blood.

Thus it can be determined that it is actually an atrium, in particularthe left atrium LA, that is being monitored. Furthermore, an optimalsignal reflecting the impedance can thereby be obtained.

The value that at the different occasions is stored in the memory 15reflects the rate of change of the impedance at the major slope 62 ofthe signal when the impedance decreases, i.e. the slope 62 reflects whenthe atrium LA is being filled with blood. As mentioned above, the valuethat is stored may for example be that maximum of the absolute of thefirst derivative dZ/dt when the impedance decreases according to theslope 62.

In order to get a more accurate value that represents the rate of changeof the impedance, the control unit 14 may be configured such that ateach occasion the steps A and B are carried out a plurality of times andsuch that from this plurality of times a representative value thatindicates the rate of change at the occasion in question is formed, andwherein it is this representative value that is stored in the memory 15.The stored value may for example be the mean value of 30 measurements,performed within two minutes, of the rate of change of the measuredimpedance.

Preferably, the control unit 14 is configured to select the differentoccasions such that the living being is likely to be in a similarphysical and/or psychological state at the different occasions. This canbe done by the fact that the control circuit 14 is configured to be ableto determine the physical and/or psychological state by means of one ormore of the following:

i) a physical activity sensor 16 which is comprised in the implantableheart monitoring device 10,

ii) the heart rate sensed or paced with the help of the implantableheart monitoring device 10,

iii) a sensed breathing of the living being,

iv) a sensed posture of the living being, for example whether the livingbeing is standing up or laying down,

v) the time of day.

It is known in the art how these different matters can be sensed ordetected.

The control unit 14 is configured such that the different occasions aredistributed over several days such that the variation of the storedvalues represents the variation in the detected rate of change during alonger period.

FIG. 6 illustrates an example of how the stored values (that representthe mentioned maximum of the absolute of dZ/dt at the occasion inquestion) vary over a number of days. In FIG. 6 it can be seen that thestored values decrease substantially after about 12 days. This is anindication of the fact that the heart condition became worse after about12 days.

The control unit 14 can be configured to create a warning message if thestored values change more than a predetermined amount over a predefinedtime period. For example, a warning message may be created if the storedvalue has decreased more than 20% since the first value was stored (i.e.if the actual last value stored is less than 80% of the first storedvalue).

The control unit 14 may also be configured such that for example 24 ofthe occasions take place during a 24 hour period such that the variationof the stored values represents the variation in the detected rate ofchange of the measured impedance during a 24 hour period. The controlunit 14 can be configured to for each day also store a value thatrepresents the variation in the detected rate of change of the measuredimpedance during each 24 hour period, and to determine how thedetermined variation changes from day to day. A warning message may forexample be created if the change, from day to day, of the determinedvariation, in the detected rate of change of the measured impedanceduring a 24 hour period, has decreased more than 20% since the previousdetermined variation in the detected rate of change of the measuredimpedance during a 24 hour period.

FIG. 7 shows a schematic flow chart illustrating a manner of carryingout a method according to the invention.

In a first step it can be determined, in the same manner as explainedabove, whether the patient is in a physical/psychological state that issuitable for carrying out the impedance measurement. If this is not thecase, the measurement is not carried out at the present moment. If,however, the state of the patient is suitable for measurement, then aQRS is detected. The impedance is measured as explained above within ashort time interval after the detected QRS.

In the next step, it is determined whether the measured impedance isoptimal. This can be done as explained above. Among other things, it isdecided whether the impedance actually decreases as expected if theimpedance represents the amount of blood in the left atrium LA.Furthermore, signal/noise etc may be detected in order to decide whetherthe measured impedance is optimal. If this is not the case, anothercombination of electrode surfaces is tested for injecting a current andfor measuring a voltage. When an optimal measured impedance (or asufficiently reliable impedance measurement) is obtained, a suitableelectrode combination has thus been decided. The method then continueswith the actual measurements that are to be monitored and stored.

If it is time for a new measurement (a new occasion), then (although notshown in the figure) it is first determined whether the patient is in asuitable state for measurement, and, if this is the case, the impedanceis measured by A) monitoring how the measured impedance varies duringthe specific portion of the heart cycle and by B) determining a valuethat represents the rate of change of the measured impedance during atleast a part (the slope 62 in FIGS. 3 and 4) of the portion of the heartcycle. In order to determine that it is the correct portion of the heartcycle that is considered, it is possible to detect for example the QRSas explained above.

A value that represents the rate of change of the impedance during saidpart of the heart cycle is stored in the memory 15. Preferably, at eachoccasion, the steps A and B are carried out a plurality of times suchthat a representative value that indicates the rate of change at theoccasion in question is stored in the memory.

In the next step, it is decided whether a warning message should becreated. The criteria for creating a warning message may be analogous tothose described above. A warning message may for example be created ifthe recently stored value that represents the rate of change of theimpedance is more than 20% lower than the first value that was stored.

When it is time for a new measurement, the described procedure isperformed again. The procedure may be performed for example once a dayin order to monitor the variation in the detected rate of change of themeasured impedance during a longer period. However, it is also possibleto perform the procedure for example every hour such that also thevariation of the stored values represents the variation in the detectedrate of change of the measured impedance during a 24 hour period. Thiscan be done at a plurality of different days such that it is determinedhow the determined variation changes from day to day.

With the present invention it is thus possible to monitor the heartcondition and to discover, at an early stage, if the heart conditionbecomes worse. The stored values and the warning messages may becommunicated to a physician, for example with the help of so-calledtelemetry or in other manners. It is also possible to arrange the devicesuch that the device automatically carries out a measure if the heartcondition becomes worse. For example, the device may in that case changea pacing routine in order to improve the function of the heart. Thedevice may also be arranged to automatically deliver a suitablepharmaceutical drug to the patient in question in response to thedetected heart condition.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim as our invention:
 1. An implantable heart monitoring systemcomprising: a control unit comprising a control circuit; a memory incommunication with said control circuit; a plurality of electrodesurfaces adapted for positioning to measure an impedance across at leasta part of an atrium of the heart to generate an impedance signal thatvaries as a function of a change in volume of said atrium; cardiacsensing circuitry in communication with at least some of said electrodesurfaces to sense cardiac events in at least one chamber of the heart;said control circuit being configured to determine a first valueindicating the rate of change of the measured impedance signal during atleast a portion of a first cardiac cycle, and to store the first valuein said memory; and said control circuit being configured to determine asecond value indicating the rate of change of the measured impedancesignal during at least a portion of a second cardiac cycle and to storethe second value in said memory, to produce, in said memory, a record ofstored values having a variation that is related to variation of a speedwith which the atrium is filled with blood during atrial diastole,wherein said control circuit is configured to determine whether saidimpedance signal decreases within a time interval following detection ofa QRS signal by the cardiac sensing circuitry to determine whether themeasured impedance indicates an inverse of an amount of blood in theatrium.
 2. An implantable heart monitoring system as claimed in claim 1wherein said control circuit is configured to determine said rate ofchange of said measured impedance as a rate of change when said measuredimpedance decreases.
 3. An implantable heart monitoring system asclaimed in claim 1 comprising a state detector in communication withsaid control circuit that supplies information to said control circuitselected from the group consisting of physical activity of the subject,the heart rate of the subject, respiration of the subject, posture ofthe subject, and time of day.
 4. An implantable heart monitoring systemas claimed in claim 1 wherein said control circuit is configured tocommunicate with at least three of said plurality of electrode surfacesto test different combinations of said at least three plurality ofelectrode surfaces for supplying current thereto or for detecting avoltage there across to identify an optimal combination of said at leastthree plurality of electrode surfaces for making said impedancemeasurement to indicate said volume change of said atrium.
 5. Animplantable heart monitoring system as claimed in claim 4 wherein saidcontrol circuit is configured to determine said optimal combination ofsaid at least three plurality of electrode surfaces from at least onecriterion selected from the group consisting of a strength of a detectedsignal, a signal-to-noise ratio of a detected signal, and occurrence ofa decrease in the detected signal in a part of said time cycle that islikely to represent filling of the atrium with blood.
 6. An implantableheart monitoring system as claimed in claim 1 wherein said controlcircuit is configured to emit a humanly perceptible warning when thevalues stored in said memory change more than a predetermined amountwithin a predetermined time period.
 7. An implantable heart monitoringsystem as claimed in claim 1 wherein said control circuit is configuredto emit a humanly perceptible warning when the rate of change of saidmeasured impedance from one of said periods to a next of said periodssatisfies at least one predetermined criterion.
 8. An implantable heartmonitoring device comprising: cardiac sensing circuitry in communicationwith at least some of said electrode surfaces to sense cardiac events inat least one chamber of the heart; a control unit comprising a controlcircuit; a memory in communication with said control circuit; saidcontrol circuit being configured to determine a first value indicatingthe rate of change of a measured impedance during at least part of afirst cardiac cycle, and to store said first value in said memory; andsaid control circuit being configured to determine a second valueindicating the rate of change of the measured impedance during at leastpart of a second cardiac cycle and to store said second value in saidmemory, to produce, in said memory, a record of stored values having avariation that is related to variation of a speed with which the atriumis filled with blood during atrial diastole, wherein said controlcircuit is configured to determine whether said impedance decreaseswithin a time interval following detection of a QRS signal by thecardiac sensing circuitry to determine whether the measured impedanceindicates an inverse of an amount of blood in the atrium.
 9. A heartmonitoring method comprising the steps of: measuring an impedance acrossat least a part of an atrium of the heart to generate an impedancesignal that varies as a function of a change in volume of said atrium;determining a first value indicating the rate of change of the measuredimpedance signal during at least a portion of a first cardiac cycle;storing said first value in memory; and determining a second valueindicating the rate of change of the measured impedance signal cardiaccycle; storing the second value in the memory wherein the first andsecond stored values provide information related to variation of a speedwith which the atrium is filled with blood during atrial diastole; anddetecting a QRS complex and determining whether said impedance signaldecreases within a time interval following said QRS complex to detectwhether the measured impedance indicates an inverse of an amount ofblood in the atrium.
 10. A heart monitoring method as claimed in claim 9comprising, communicating with at least three electrode surfaces to testdifferent combinations of said at least three electrode surfaces forsupplying current thereto or for detecting a voltage there across toidentify an optimal combination of said at least three electrodesurfaces for making said impedance measurement to indicate said volumechange of said atrium.
 11. A heart monitoring method as claimed in claim9 comprising, emitting a humanly perceptible warning when the rate ofchange of said measured impedance from one of said periods to a next ofsaid periods satisfies at least one predetermined criterion.